The Super Audio Compact Disk (SACD) system records audio data on an optical disk as a single-bit digital data stream at a high oversampling rate. This high oversampling rate advantageously extends the signal bandwidth well beyond the range of human audibility and reduces the need for significant anti-aliasing filtering. Consequently, audible time-domain effects, which normally result when steep low-pass anti-aliasing filters are used in traditional digital audio systems, are typically no longer a significant problem in SACD systems.
The advantages provided by the high oversampling rate of the SACD bit stream are countered to a certain degree by the significant disadvantages of the one-bit data format. For example, to maintain a large dynamic range in the audio band using one-bit data, the quantization noise must be shifted out of the audio band with a noise transfer function having a relatively steep passband edge. Delta-sigma modulators are commonly utilized in SACD systems to generate such a noise transfer function, although conventional delta-sigma modulators are normally insufficient for some advanced audio applications.
Increasingly, SACD systems are being integrated into audio systems, such as those found in home theater systems, which utilize a set of main speakers without an extended bass response and a subwoofer which provides the remaining low frequency bass output. The task of digitally splitting and converting to analog signals the bass and higher frequency data in these systems is difficult since highly oversampled data is being processed. Ideally, the crossover filtering and mixing required to make the frequency split would be done at the full SACD oversampling rate to realize the advantages of highly oversampled data discussed above. Filtering from highly oversampled one-bit data, however, normally requires performing highly accurate multiplications on digital data words of significantly long length. Accurate multiplication of long digital words, in turn, becomes computationally intensive in either hardware or software.
Hence, some new techniques are required for processing highly oversampled audio data, such as SACD data, which support applications such as home theater audio and, at the same time, are relatively simple and inexpensive to implement.